These will be useful if one wants to put additional things on the bars.

Noted in #30224 (comment), which this will simplify because instad of

    def center_top(rect):
        return rect.xy[0] + rect.get_width()/2, rect.xy[1] + rect.get_height()

    xy = np.array([center_top(r) for r in container.patches])
    ax.errorbar(xy[:,0], xy[:,1], yerr=yerr)

we can then simply do

    ax.errorbar(container.position_centers, container.tops, yerr=yerr)

Naming: The naming has to work for horizontal and vertical bars.

• position_centers: I use position as the non-data coordinate. This is consistent with the naming in https://matplotlib.org/devdocs/api/_as_gen/matplotlib.axes.Axes.grouped_bar.html (bar()/barh() use x/y, which doesn't generalize to orientation.
  I intentionally chose position_centers over center_positions, because center_position could be misinterpreted as the (x, y) coordinates of the centers of the bar rectangles.
• tops: https://matplotlib.org/devdocs/api/_as_gen/matplotlib.axes.Axes.grouped_bar.html universally uses heights and the upper end of the height is reasonably the top. I think its bearable to have this naming also in the face of potentially horizontal bars.